To drive a plurality of hydraulic actuators by a single hydraulic pump, such a parallel circuit type hydraulic apparatus A as shown in FIG. 7 has commonly been used.
In the hydraulic apparatus A, hydraulic oil discharged from a hydraulic pump B is fed to a first hydraulic actuator D1 through a first actuating valve C1 and to a second hydraulic actuator D2 through a second actuating valve C2.
However, the above-mentioned arrangement of the hydraulic apparatus A has such a drawback that if the hydraulic oil is simultaneously fed to the plurality of hydraulic actuators D1 and D2, then the quantity of hydraulic oil fed to a lower load side hydraulic actuator becomes larger which results in that a higher load side hydraulic actuator is not supplied with a sufficient quantity of hydraulic oil.
FIG. 8 shows a hydraulic apparatus which has been proposed to obviate the drawback mentioned above. In this hydraulic apparatus A', a first and a second pressure compensating valves E1 and E2 are interposed between the first actuating valve C1 and the first hydraulic actuator D1 and between the second actuating valve C2 and the second hydraulic actuator D2.
Inlet side pressures of the first and second pressure compensating valves E1 and E2 are applied as pilot pressure to the flow rate increasing side pressure receiving surfaces of the spools in the respective pressure compensating valves E1 and E2, and output pressure from a shuttle valve F interposed between a hydraulic passage extending from the first pressure compensating valve E1 to the first hydraulic actuator D1 and a hydraulic passage extending from the second pressure compensating valve E2 to the second hydraulic actuator D2, is applied as pilot pressure to the flow rate decreasing side pressure receiving surface of the respective spools.
With the foregoing hydraulic apparatus A', the maximum hydraulic pressure at the higher load side hydraulic actuator D1 or D2 is permitted to act on the flow rate decreasing side pressure receiving surfaces of the pressure compensating valves E1, E2 under the action of the shuttle valve F, so that the flow rate of hydraulic oil at that one of the pressure compensating valves which is coupled to the higher load side hydraulic actuator, is restrained, while the flow rate of hydraulic oil at that one of the pressure compensating valves which is coupled to the lower load side hydraulic actuator, is increased.
Thus, even if the first and second hydraulic actuators D1 and D2 are loaded differently, a quantity of hydraulic oil which is proportional to the hydraulic passage opening area, i.e., the extent of lever actuation in the respective actuating valve C1, C2, is distributed to the respective hydraulic actuator D1, D2, irrespective of the difference in load between the hydraulic actuators.
In the above-described hydraulic apparatus A', the outlet port side pressure of the pressure compensating valve is permitted to act on the flow rate decreasing side pressure receiving surface of the spool therein, and outlet side pressure P3 is caused to be lower than the inlet side pressure P2 of the valve due to pressure loss which tends to be caused when the hydraulic oil passes through the pressure compensating valve.
The flow rate Q1 in the lower load side pressure compensating valve and the flow rate Q2 in the higher load side pressure compensating valve are given as follows: ##EQU1## where C is a constant, and a1 and a2 are the opening areas of the respective actuating valves.
In effect, an error corresponding to the pressure loss (P2-P3) in the pressure compensating valve is induced in the quantity of hydraulic oil distributed to each hydraulic actuator.
The drawback mentioned just above can be eliminated by causing the inlet port side pressure of the pressure compensating valve to act on the flow rate decreasing side pressure receiving surface of the valve; however, there arises such a problem that the pressure compensating valve tends to be erroneously operated by flow force occurring within the pressure compensating valve due to the fact that the inlet port side pressure P2, i.e., an equal pressure is permitted to act on the flow rate increasing side and flow rate decreasing side pressure receiving surfaces of the spool in the valve. More specifically, if the above-mentioned flow force acts in such a direction as to close the pressure compensating valve, then the inlet port side pressure P2 of the pressure compensating valve becomes higher than the outlet port side pressure P.sub.3, and thus power loss is caused.
In view of the above-described state of art, it is a first object of the present invention to provide a hydraulic apparatus capable of preventing malfunction of pressure compensating valves, and distribution hydraulic oil to a plurality of hydraulic actuators with a proper flow rate corresponding to the extent of actuation of actuating valves.
As the actuating valves C1, C2 in the hydraulic apparatus of FIG. 8, three-way change-over valves are employed to permit the hydraulic actuators D1, D2 to be reversibly operated, the change-over valves arranged, at neutral position, to connect the pressure compensating valves E1, E2 in communication with a drain tank.
Thus, when the actuating levers of the actuating valves C1, C2 are made to assume neutral position, the hydraulic oil in the inlet side hydraulic passages of the pressure compensating valves E1, E2 is drained so that the spools are returned to their initial positions by holding pressures of the hydraulic actuators D1, D2.
Consequently, when the actuating lever is moved from the neutral position to the operating position, part of hydraulic oil discharged from the actuating valves C1, C2 is used to cause the spools of the pressure compensating valves to be displaced to a proper compensating position so that buildup of the maximum pressure provided by the shuttle valve F is delayed correspondingly, which leads to a reduction in the response of the hydraulic actuator to lever actuation.
In view of such a state of art, it is a second object of the present invention to provide a hydraulic apparatus capable of improving the response of hydraulic actuators to lever actuation of actuating valves.
In the hydraulic apparatus A' arranged as mentioned above, when the actuating levers of the actuating valves C1, C2 are simultaneously actuated with a maximum stroke, there arises such a problem that the maximum operating speed of the hydraulic actuators is decreased as compared with the parallel circuit type hydraulic apparatus A shown in FIG. 8.
More specifically, in case where the maximum quantity of hydraulic oil supplied from the hydraulic pump B is less than the sum of the quantities of hydraulic oil which are required by the respective hydraulic actuators D1, D2 when the levers are fully actuated, with the aforementioned parallel circuit type hydraulic apparatus A, more hydraulic oil is fed to the lower load side hydraulic actuator so that the maximum operating speed of the hydraulic actuators in the hydraulic apparatus A is maintained at a high value, whereas with the aforementioned hydraulic apparatus A' provided with pressure compensating valves, a limited quantity of hydraulic oil from the pump B is evenly distributed to the respective hydraulic actuators D1, D2 so that the maximum operating speed of the hydraulic actuators is reduced.
The above-mentioned phenomenon constitutes a cause for a machine using the hydraulic apparatus A' having the above construction to impart an uncomfortable feeling in terms of operation to an operator who is experience in operating a machine adopting the parallel circuit type hydraulic apparatus A such as power shovel or the like, for example.
In view of such a state of art, it is a third object of the present invention to provide a hydraulic apparatus capable of restricting the quantities of hydraulic oil supplied to the respective hydraulic actuators from becoming improper and providing a good operational feeling to an operator.